Portland can do it if they want, their issue is that the ground is extremely soggy year round, with a underground stream running by the stadium site. That, plus the normal Cascadia rainfall means it would be very difficult to maintain the surface to the quality required.
They are looking into it, and might do it if they can find some technical solutions to things, but for not they have turf with extra padding underneath which makes theirs a really good surface to play on.
You rarely hear people complaining about portland's surface. Compared to Seattle, Vancouver or New England it's loads better.
****, I forgot the NE situation